In recent years, insulated gate bipolar transistors (IGBT), diodes, and the like have been used as semiconductor devices used in power conversion devices such as inverters. Diodes are conventionally connected to IGBTs in an anti-parallel manner and are used as refluxing diodes. Because of this, the diodes are sometimes referred to as free wheeling diodes (FWD).
To improve characteristics of power conversion devices such as inverters, it is critical that FWD characteristic improvements are performed in parallel with IGBT characteristic improvements. Important characteristics of the FWDs include an ON voltage (that is, a voltage drop in a conduction state), a recovery time (that is, an annihilation time of a recovery current upon recovery), a safe operation area upon recovery (that is, a region not destroyed even if a voltage is applied in a state where the recovery current is flowing), and the like. Moreover, a smaller current and voltage oscillation during recovery is more desirable. Among these, widening the safe operation area during recovery while shortening the recovery time is critical.